To make the glaze, heat oil in a large frying pan over medium heat and sauté onions and garlic for 10 minutes, or until onion is soft. Add remaining glaze ingredients and bring to a boil. Reduce to a simmer, then cover and cook for 15 minutes, or until richly flavored and reduced to 4 cups (1 liter). Transfer to a food processor or blender and process until smooth. Measure out 2 cups (500 ml) to use as a glaze; set aside remainder to serve with the chicken.